1. Field of the Invention
The present invention relates to a high voltage semiconductor device, and in particularly to a high voltage semiconductor device in which an IGBT or a power MOSFET and a free wheel diode are formed in a single semiconductor substrate.
2. Description of the Background Art
Recently, from the viewpoint of energy saving, inverter circuits have been widely used. Inverter circuits control electric power for home electric appliances, industrial electric power equipment, or the like. An inverter circuit switches ON and OFF of voltage or current using a power semiconductor device embedded in the inverter circuit. Examples of the power semiconductor device include an Insulated Gate Bipolar Transistor (IGBT), a power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), and the like.
The inverter circuit drives an inductive load such as an induction motor. The inductive load generates a counter-electromotive force. The inverter circuit requires a free wheel diode. The free wheel diode causes a current to flow in a direction opposite to a direction in which a main current of an IGBT or the like generated by the counter-electromotive force flows.
In a typical inverter circuit, an IGBT or the like and a free wheel diode as separate components are connected in anti-parallel. In order to provide a small-sized and lightweight inverter device, high voltage semiconductor devices in which an IGBT or the like and a free wheel diode are formed into one chip (integrated) have been developed (see Japanese Patent Laying-Open Nos. 04-192366, 2004-363328, and 2007-227982, and U.S. Patent Application Publication No. 2009/0140289). In a high voltage semiconductor device formed into one chip, for example, a collector region of the IGBT and a cathode region of the free wheel diode are formed on a back surface side of a semiconductor substrate.
Unless the collector region of the IGBT and the cathode region of the free wheel diode are formed on the back surface side of the semiconductor substrate with being separated sufficiently, a snap-back phenomenon occurs. If the collector region of the IGBT and the cathode region of the free wheel diode are formed on the back surface side of the semiconductor substrate with being separated sufficiently, an effective area of the collector region of the IGBT is decreased, and a chip area is increased. A decrease in the effective area of the collector region of the IGBT causes deterioration of performance of the high voltage semiconductor device. An increase in the chip area causes an increase in the cost of manufacturing the high voltage semiconductor device.
It is assumed that, in order to avoid deterioration of performance and increase in the manufacturing cost of the high voltage semiconductor device, the collector region of the IGBT and the cathode region of the free wheel diode are formed on the back surface side of the semiconductor substrate without being separated sufficiently. In this case, occurrence of a snap-back phenomenon is suppressed by forming a separating portion such as a trench having an insulator embedded therein between the collector region of the IGBT and the cathode region of the free wheel diode. However, in order to form the separating portion such as a trench having an insulator embedded therein, it is necessary to form a deep groove in a thickness direction of the semiconductor substrate. As a result, the cost of manufacturing the high voltage semiconductor device is increased.